The invention relates, generally, to cryogenic liquid storage and delivery systems and, more particularly, to an improved storage tank for such liquids.
As is well known, cryogenic liquids, such as liquid natural gas (LNG), nitrogen, oxygen, CO.sub.2, methane, hydrogen and the like, are liquified gasses that exist at extremely cold temperatures. Special problems are encountered in handling cryogenic liquids because of the extremely cold temperatures required to maintain the liquid phase. One such problem is encountered in single line filling of storage tanks. While the tank of the invention can be used with virtually any cryogenic liquid, reference will be made specifically to LNG to facilitate explanation of the invention.
Presently tanks are filled by spraying sub-cooled LNG into the top of the tank via a spray head thereby to condense the gas in the tank and collapse the pressure head therein. When the pressure head collapses, the pressure in the tank is substantially reduced such that the cryogenic liquid can freely flow into the tank. The cryogen will continue to flow into the tank until the level of the cryogen reaches and submerges the spray head. Once the spray head is submerged, the pressure in the tank will gradually increase because the incoming sub-cooled cryogen is no longer in direct contact with and therefore unable to collapse the pressure head as quickly. However turbulance in the tank keeps the liquid from stratifying therefore the vapor space continues to collapse until the tank is liquid full. A pressure monitor is placed in the fill line to automatically shut-off the flow of cryogen when the pressure in the tank reaches a predetermined value corresponding to the desired fill level of cryogenic fluid. The head or ullage space, i.e. the area between the top of the liquid cryogen and the top of the tank, accommodates the vaporizing cryogen and increases the hold time of the tank before venting occurs.
While the above-described fill system is workable, it requires a very sensitive and expensive pressure monitoring device to determine when the spray head is submerged because the pressure rise is relatively small. If the pressure monitor does not detect the pressure rise, the tank will completely fill with liquid. A liquid full tank is undesirable for a number of reasons including the reduction of the hold time of the tank to near zero because there is no head or ullage space to accommodate the vaporizing liquid cryogen.
Thus, an improved filling system for cryogenic fluids is desired.